When a well drilled for oil prospecting reaches a given depth, a casing is generally inserted down the well to said depth and cement is injected into the annular space surrounding the casing so as to hold and otherwise support the casing in place and also to isolate the various formations through which the well passes from one another so that fluid migration does not occur. For example, it would be detrimental to production if fluid were to pass from a water-bearing formation to an oil-bearing formation along an un-cemented or insufficiently cemented casing. During the cementing operation, the cement is usually injected/introduced into the wellbore through the casing and subjected to pressure whereupon the cement will flow out of the bottom of the casing and then upwardly within the annular space between the casing and the formation. Many cementing protocols involve emplacing cement at least 500 feet above the flow zone.
Effectively placing cement in the annular space located between casing and formation is a challenging operation. The drilling mud originally in place in the annulus at the end of the drilling phase must be fully displaced and replaced by the cement slurry. One challenge is that the total volume of the annulus and thus the amount of cement slurry needed for a particular casing job is often not well characterized. Another challenge comes from the fact that the annulus is not concentric: despite the use of centralizers, the casing string will lean towards the bottom of the hole in deviated or horizontal wells. A second challenge is related to the nature of the drilling mud and cement slurry: they are both viscoplastic and exhibit a yield stress. As a result they can remain unyielded in the narrow part of the eccentric annulus, thus preventing correct placement of the cement slurry. Further complicating matters is that during cement placement losses are often encountered, resulting in drilling fluid and cement to leak off into the formation and therefore not be properly placed.